There has been a great deal of interest in developing better and more efficient methods for storing energy for applications such as cellular communication, satellites, portable computers, and electric vehicles to name but a few. Accordingly, there have been recent concerted efforts to develop high energy, cost effective batteries having improved performance characteristics.
Rechargeable or secondary cells are more desirable than primary (non-rechargeable) cells since the associated chemical reactions which take place at the positive and negative electrodes of the battery are reversible. Electrodes for secondary cells are capable of being recharged by the application of an electrical charge thereto. Numerous advanced electrode systems have been developed for storing electrical charge. Concurrently much effort has been dedicated to the development of electrolytes capable of enhancing the capabilities and performance of electrochemical cells.
Heretofore, electrolytes have been either liquid electrolytes as are found in conventional wet cell batteries, or solid films as are available in newer, more advanced battery systems. Each of these systems have inherent limitations and related deficiencies which make them unsuitable for various applications. Liquid electrolytes, while demonstrating acceptable ionic conductivity tend to leak out of the cells into which they are sealed. While better manufacturing techniques have lessened the occurrence of leakage, cells still do leak potentially dangerous liquid electrolytes from time to time. Moreover, any leakage in the cell lessens the amount of electrolyte available in the cell, thus reducing the effectiveness of the device.
Solid electrolytes are free from problems of leakage, however, they have traditionally offered inferior properties as compared to liquid electrolytes. This is due to the fact that ionic conductivities for solid electrolytes are often one to two orders of magnitude poorer than a liquid electrolyte. Good ionic conductivity is necessary to insure a battery system capable of delivering usable amounts of power for a given application. Most solid electrolytes have not heretofore been adequate for many high performance battery systems.
One class of solid electrolytes, specifically gel electrolytes, have shown great promise for high performance battery systems. Gel electrolytes contain a significant fraction of solvents and/or plasticizers in addition to the salt and polymer of the electrolyte itself. Today, most electrochemical cells based on a lithium-ion chemistry employ at least some graphite in the anode of the cell. In such cells, graphite rapidly loses much of its capacity in the presence of the best high boiling point, low melting point electrolyte solvents, examples of which include propylene carbonate (PC). Since low melting point solvents are needed to obtain low-temperature performance, a trade-off is made by incorporating a significant amount of low-melting, low-boiling point solvent instead of PC. Unfortunately, many of these alternative solvents are flammable and/or combustible.
Accordingly, there exists a need for a new electrolyte system which combines the properties of high ionic conductivity and excellent low temperature performance, all while using a non-volatile solvent or plasticizer. The electrolyte should not compromise performance at higher temperatures, should be relatively cost effective, and should be easy to produce.